The conventional process for preparing monoethylene glycol (EG) entails partial oxidation of ethylene followed by hydration of the resulting oxide. Hydrogenation of glycolic acid and its esters (hereafter collectively referred to as glycolates or glycolate esters) is a substitute technology for producing EG.
Homogeneous ruthenium complexes bearing the tripodal phosphine ligand 1,1,1-tris-(diphenylphosphinomethyl)ethane, commonly known as ‘Triphos’, are known to reduce glycolate esters under elevated temperature and hydrogen pressure. Since these complexes are thermally stable, product purification by distillation followed by recycle of the resulting catalyst enriched heel would provide a process cost savings. One consequence of this approach, however, is the ruthenium catalyzed degradation of the ethylene glycol. Indeed, according to the principal of microscopic reversibility, a hydrogenation catalyst is also capable of dehydrogenation, particularly under non-reducing atmospheres.
Dehydrogenation of ethylene glycol produces hydrogen and an equivalent molar amount of glycolaldehyde which, in the presence of excess EG, forms mono- and di-EG acetals (hereafter referred to as ‘glycolaldehyde acetals’ or ‘glycolaldehydes’). Butane tetra-alcohol byproducts such as threitol and erythritol (hereafter collectively referred to as ‘by-product tetrols’) are believed to form from these glycolaldehydes. These by-product tetrols can decompose into other byproducts such as 1,2-butanediol and 1,2-propanediol (hereafter collectively referred to as ‘by-product diols’).
There exist downstream locations where the reaction mixture is under non-reducing atmospheres and additional by-products can form, such as in heat trace lines leading to a distillation column, and in the purification section (a distillation column itself) that is used to separate EG from the reaction mixture effluent of the hydrogenation reactor. The concentration of these by-products, particularly that of the by-product diols, can be problematic during product purification, especially in separation via distillation. The by-product diols in particular will tend to travel with EG as an overhead vapor, thereby reducing the purity of the EG overhead. While the separation of EG from by-product tetrols and by-product glycolaldehyde species is easier to separate than EG from diols, the formation of any by-products while the effluent from the hydrogenation reactor is in route to the purification section, or their formation in the distillation column, represents a yield loss in EG and variability in EG yield. This is also undesirable in a commercial hydrogenation/purification EG process.
We have found that dehydrogenation or degradation reactions of ethylene glycol can also occur in a reducing atmosphere to form by-products such as diols other than ethylene glycol, by-product tetrols, and by-product glycolaldehyde species. Even under hydrogenation conditions, the EG being formed can be subjected to dehydrogenation to produce by-products. During hydrogenation in the hydrogenation reactor, one can control the amount of by-product formation in the hydrogenation reactor through temperature, pressure, degree of conversion, and activity of the catalyst and one can also account for the number of moles of diols produced during the hydrogenation step. However, the formation of by-products after the hydrogenation step is problematic because the formation of these additional by-product diols are difficult to account for, which adds to variability and reduced control of the process, and also adds to the quantity of by-products already present in the hydrogenation reactor and yield loss of EG. Thus, minimizing all by-product formation downstream of the hydrogenation reactor, and especially by-product diol formation, and minimizing variability in the compositional mixture after discharging the effluent from the hydrogenation reactor would clearly benefit a commercial application of a hydrogenation/purification EG process.